Synecdoche is a figure of speech where part of something names the whole;
A sentence with an example of a Synecdoche is:
“He led 50 Head down the dusty trail.” Standing for 50 Head of Cattle.
A Metonymy is a figure of speech where one thing or concept is not called by its own name.
A sentence with an example of a Metonymy is:
Will you please approach the Bench? In this sentence “Bench” is used as a substitute for (The Judge).
Metonymy may also be compared to a Metaphor. Both involve the substitution of one thing for another. In metaphor, the substitution is based on some similarity, where, in metonymy, the substitution is based on an understood association.